LONDON --
Balancing power supply and demand is always a complex process. When large volumes of renewables such as solar PV, wind and tidal energy, which can change abruptly with weather conditions, are integrated into the grid, this balancing process becomes even more difficult.

The issue for power plants is flexibility. "Large amounts of wind energy are being reliably and cost-effectively integrated onto the power system today," said Denise Bode, CEO of the American Wind Energy Association (AWEA), who added "Energy storage can be a valuable resource for the power system in maximising the efficient use of this resource, and add flexibility for electric utilities."

The Electric Reliability Council of Texas Inc. (ERCOT) faced the renewable power industry's most critical issue in February 2008. With a huge wind portfolio in the state the wind died down, and ERCOT declared emergency conditions after a 1200-MW drop in production. The three-hour shortfall, accompanied by increasing overall electricity loads, very nearly caused rolling blackouts. David Crane, president and CEO of New Jersey-based NRG Energy Inc., told the Houston Business Journal that "If a system can go unstable in the winter because 1500 MW of expected wind turns into 400-MW wind and then fossil has to scramble to come online - and several of our plants had to scramble to fill the gap - that's a big issue and there's going to be a big debate."

Effective energy storage can match total generation to total load precisely on a second by second basis. It can load-follow, adjusting to changes in wind and solar input over short or long time spans, as well as compensating for longterm changes. While fossil plants may take 10 minutes or more to come online, and will consume fuel even on "spinning reserve" standby, storing renewable energy for later use effectively produces no emissions.

"Grid-scale storage is here now," says Ed Cazalet of MegaWatt Storage Farms, which develops and operates large electricity storage facilities that connect directly to the wholesale electric grid. "Storage should be deployed now at the gigawatt scale...where capacity, ancillary services and energy time-shifting are clearly needed," he adds. But each power plant faces different issues, and each requires a tailored energy storage solution.

Some well-established technologies offer significant energy storage capacity but require specific geographical features and considerable infrastructure. Others can be deployed rapidly to wherever they are required, but currently offer restricted capacity, often at high cost. One technology that is now attracting considerable interest is large-scale battery storage.

BATTERIES FOR LARGE-SCALE ENERGY STORAGE

Several types of batteries are used for large-scale energy storage. All consist of electrochemical cells though no single cell type is suitable for all applications.

Invented in 1859, lead-acid batteries use a liquid electrolyte and are still in common use. They store rather small volumes of energy but are reliable and, above all, cheap. In renewable energy systems multiple deep-cycle lead-acid (DCLA) batteries, which provide a steady current over a long time period, are connected together to form a battery bank. Indeed, banks of up to 1 MW of lead-acid batteries are already being used to stabilise wind farm power generation. For instance, Trojan Battery Company's industrial line of deep-cycle batteries is designed for backup and peak shifting in off-grid and grid-tied photovoltaic (PV) systems.

In a dry cell battery, the electrolytes are contained in a low-moisture paste. Lithium-ion (li-ion) batteries in particular are the subject of much interest as they have a high energy density, and larger-scale production due to emerging electric vehicle applications is expected to bring down their cost significantly.

A123 Systems is involved in a demonstration project with Southern California Edison which will see 32 MWh of li-ion battery capacity integrated with wind turbines in the Tehachapi region 100 miles (160 km) north of Los Angeles.

Mitsubishi Heavy Industries is beginning tests of a 40-foot (12 metre) long container unit housing more than 2,000 li-ion rechargeable batteries. According to the company, the system has a capacity of 408 kWh - enough to supply 100 households for three to eight hours and is designed to have a system efficiency of 90 percent. As well as grid power stabilisation, the system will be tested for micro-grid use in areas where regular grid connections are difficult, providing a stable supply of electricity from renewable sources.

Flow batteries are emerging energy storage devices that can serve many purposes in energy delivery systems. They can respond within milliseconds and deliver significant quantities of power. They operate much like a conventional battery, storing and releasing energy through a reversible electrochemical reaction with an almost unlimited number of cycles. The active chemicals are stored in external tanks, and when in use are continuously pumped in a circuit between the reactor and tanks. The great advantage is that electrical storage capacity is limited only by the capacity of the tanks.

Vanadium Redox Batteries (VRBs) are a particularly clean technology, with high availability and a long lifecycle. Their energy density is rather low - about 40 Wh per kilogram - though recent research indicates that a modified electrolyte solution produces a 70 percent improvement in energy density. Vanadium prices are volatile, though, with the increased demand for battery use likely to stress supply. Prudent Energy has installed a 1-MWh VRB energy storage system, sized at a rated power of 500 kW and a peak pulse power of 750 kW, for the China Electric Power Research Institute (CEPRI) in Zhangbei, Hebei province. Research group the Fraunhofer Institute is developing a giant 20-MWh flow battery. A 20-kW facility is planned to be operational next year; and the research team hopes to cross the megawatt threshold within five years.

Researchers at Case Western Reserve University are using iron to create a scalable energy storage system that can service a single home or an entire community. Robert Savinell, professor of chemical engineering at Case Western, calls it the rustbelt battery. Since the cost of iron is as little as 1 percent of that of vanadium, the iron-based battery is estimated to cost US$30/kWh, well below a $100/kWh goal set by Sandia National Laboratories. A large-scale 20-MWh iron-based flow battery would require two storage tanks of about 250,000 gallons (950 m3), and could supply the power needs of 650 homes for a day. The US Department of Energy's Office of Electricity Delivery and Energy Reliability is funding the research with a grant of some $600,000.

Molten salt batteries (or liquid sodium batteries) offer both high energy density and high power density. Operating temperatures of 400-700¬∞C, however, bring management and safety issues, and place stringent requirements on the battery components. In 2010, Italy's Enel opened the 5-MW Archimedes solar farm, the first in the world to use molten salt technology.

Ultracapacitors (or supercapacitors) store energy electrostatically by polarising an electrolyte, rather than storing it chemically as in a battery. Ultracapacitors have a lower energy density but a higher power density than standard batteries: they store less energy (around 25 times less than a similarly sized li-ion battery) but can be charged and discharged more rapidly.

Although ultracapacitors have been around since the 1960s, they are relatively expensive and only recently began being manufactured in sufficient quantities to become cost-competitive. Ultracapacitors have applications in 'energy smoothing', momentary-load devices, vehicle energy storage, and smaller applications like home solar energy systems where extremely fast charging is a valuable feature.

The US Advanced Research Projects Agency-Energy (ARPA-e) sponsors a number of interesting energy storage projects currently in the research and development stage, including metal-air ionic liquid (MAIL) batteries, planar sodium-beta batteries, high energy density lithium batteries, zinc-manganese oxide batteries, a liquid metal battery called Electroville and thermal energy storage with supercritical fluids. Each of these research projects promises to deliver low-cost, sustainable, high density energy storage for renewable energy applications.

COMPARISONS AND PREDICTIONS

The US Electric Power Research Institute's (EPRI) paper entitled Electricity Energy Storage Technology Options states that sodium-sulphur batteries (a variety of molten salt battery) are currently the third most widely used energy storage solution, with 316 MW installed worldwide.

The Electricity Storage Association (ESA) compared energy storage technologies for high power and high energy applications, as illustrated in Table 1 (below). Each technology has inherent limitations or disadvantages that make it practical or economical for only a limited range of applications.

Frost & Sullivan's analysis of the European utility-scale battery market found earned revenues of $126.4 million in 2010 and the company says it expects this to increase to $564.9 million in 2015.

Li-ion batteries will grow from $795 million in revenue in 2011 to $2.2 billion in 2016, the report states, adding that thanks to its improved cycle life and energy density over lead-acid batteries, li-ion will see narrow penetration into the high-end datacenter market. If li-ion developers can trim costs 33 percent to $400/kWh and demonstrate improved lifetimes, the technology could usurp further market share.

Operations
One of the world's biggest electric "batteries", Ludington can provide energy at a moment's notice. Its ability lies in its 27-billion gallon reservoir and a set of six turbines that drive electric generators. Those same turbines double as giant water pumps to fill the reservoir with water from Lake Michigan.

At night, when electric demand is low, Ludington's reversible turbines pump water 363 feet uphill from Lake Michigan. The water is pumped through six large pipes, or "penstocks", to the 842-acre reservoir. During the day, when electric demand is high, the reservoir releases water to flow downhill through the penstocks. The flowing water turns turbines and generators in the powerhouse to make electricity.

The plant can generate up to 1,872 megawatts — enough electricity to serve a community of 1.4 million residential customers. The output is more than double the capacity of any single unit on Consumers Energy's system.

Ludington's relatively simple technology enables the plant to respond quickly to the daily, weekly and seasonal highs and lows of Michigan's energy demand. The plant also saves customers money by enabling Consumers Energy to avoid the expensive spot market when customer demand exceeds the capacity of the company's baseload plants. The immense size of Ludington and its six-unit design offers flexibility in balancing customer demand with electric output on a moment's notice.

Ludington has won several national and company awards for design and safe operation. Contractor and company personnel involved in the design and construction of the plant return periodically to Ludington in a special reunion for the project that many call the highlight of their careers

This is great our company builds large format battery systems. goto:http://usunitedenergysystems.com/6199/47175.html

MASS Energy Storage Unit for Large Energy Production Companies, Stores Surplus Energy at the low demand hours to be released at high peek demand hours, This will make the DOE Smart Grid Program become reality and not just a dream. Our future is here and now not years away. The commodity traders will love this now that they can trade stored power, buy cheap at night and sell at peek demand value.

"The author should draw the distinction between molten salt as a heat storage medium, using its high latent heat of fusion (as in the Archimede 5 mW solar farm), and molten salt batteries, where the salt is used as the electrolyte in a high temperature electrochemical battery. e.g sodium/sulphur battery. Very different methods of storing energy!"

geoffrey-gunning-39130, thanks very much for pointing this out. You're absolutely correct, of course, and my apologies for the ambiguity.

Maybe wind turbine towers could be used to store the large volume of flow battery iron or vanadium based electrolyte and directly using excess wind power to regenerate the electrolyte. Another option is to use the tower to store water in an upper tank and use this to generate hydro electic power combined with using excess wind power to pump water back from a lower tank to the upper tank. I estimate that only between 20 and 50 kWh of hydro electric energy may be stored in a large wind turbine tower and therefore this option may not be economically viable.

What went missing in Texas was ~3000 MWh of power == 10,800 GJ. Something that 11M cu.m. of water in deep storage could have fixed. Texas has ~24,000,000,000 cu.m of managed water reserves (~2/3 of max capacity). Raising 4% of this by 1 meter (or 0.4% by 10 m) would have been sufficient.

The liquid metal battery technology being developed at Prof. Dan Sadoway's lab at MIT looks like it probably has the best chance of meeting the criteria for cost effective, environmentally friendly utility scale energy storage. If I were a billionaire, I'd definitely be investing in bringing this technology to market:
http://web.mit.edu/newsoffice/2009/liquid-battery.html

The balancing of power is a huge issue in integrating renewable energy sources into the grid. There is only one solution I have seen that stores AC power as AC power, and that is the RPM (Ring Power Multiplier). This NASA-tested technology offers continuous, instantaneous power protection, and power conditioning for the grid. And it would work in conjunction with batteries. I expect we will see more about this soon, now that they are working with ORNL.
http://www.prweb.com/releases/RPM/ringpowermultiplier/prweb8750521.htm

My understanding is (and I could have been fed a line by an enthusiastic greenie) that solar thermal reflection farms pointed at a focal tower generate temperatures that make hydrogen extraction from water a no-brainer.

You can store hydrogen for as long as you like. Transport it to wherever you like. Does not involve altering the flow of a river... but has a minor boom boom risk.

ray: while the amount of weight required per kWh stored is not a big consideration for stationary facilities, it is still a consideration, and you have to lift a lot of weight, or lift it a long way, for the levels of power we are talking about here. If you want to use kinetic storage, you're better off blowing up balloons under deep water and capturing the energy as they rise -- or with flywheels.

I have wondered why I have never seen a plan for using weight as the storage medium. Using weight that is lifted to store energy and lowering it to recover energy using cables or gears seems cheap and efficient. Additionally they require no expensive chemicals, there is no water needed, they can be located anywhere, there is little environmental impact, it scales extremely well and the technology is well known. Perhaps it is overlooked because it is so simple. Perhaps there is another reason that you can enlighten me about.

Another example of hydro stored power would be Castaic Pumped Storage Power Plant: http://v16.lscache6.c.bigcache.googleapis.com/static.panoramio.com/photos/original/36916952.jpg

It does the same thing as battery power in that energy is stored as potential energy when it is pumped up to Pyramid Lake. When power is needed, it is 'discharaged' by generating power by flowing water from Pyramid to Castiac lake through the hydro-turbine generators.

Wind power may be greater at night than during the day. Since the demand would be less at night, the 'excess' capacity can be stored to be used at a time when demand is higher.

Pumped hydro has it's appeal; however, one should not underestimate the complexities in operating hydro-electric reservoirs. In most cases, power generation is only one of a number of functions and responsibilities. One serious consideration is that water quality must be maintained. This includes downstream flows, reservoir and outflow water temperatures and oxygen levels, water levels, etc. Recycling discharge water can have a negative impact on all of these things. The simplest approach is virtual storage where hydro-electric production is dispatched in opposite phase to other generators but even this can create water quality issues plus one large economic issue - the operators of hydroelectric projects would not generally appreciate having their revenue stream curtailed.

The Germans have an approach that is likely less problematic where they use the elevation differential of non-producing mines for hydraulic storage - in addition to being able to produce very large head pressure, this can be done entirely underground minimizing land use and environmental concerns.

Open air pumped storage systems can only avoid some of the water quality issues. It's pretty much impossible to prevent fish, fowl and game from utilizing a large surface reservoir which automatically brings with it certain obligations (just ask any prairie farmer with a dugout).

"Molten salt batteries (or liquid sodium batteries) offer both high energy density and high power density. Operating temperatures of 400-700¬?C, however, bring management and safety issues, and place stringent requirements on the battery components. In 2010, Italy's Enel opened the 5-MW Archimedes solar farm, the first in the world to use molten salt technology."

The author should draw the distinction between molten salt as a heat storage medium, using its high latent heat of fusion (as in the Archimede 5 mW solar farm), and molten salt batteries, where the salt is used as the electrolyte in a high temperature electrochemical battery. e.g sodium/sulphur battery. Very different methods of storing energy!

More good reasons why rooftop - consumer owned solar and the age of the electric car will go hand in hand. Charging the electric car and the swappable batteries will assist the utility companies. Direct energy from the sun to the consumer raises the overall efficiency. Large companies, instead of trying to usurp control of America's power needs, should embrace the fact that distributed solar rooftops and carports are providing the infrastructure necessary for growth.

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